Integrated electronic circuits are typically prepared by sequentially depositing patterned thin film layers, e.g., of connectors, contacts and conductors, atop a crystalline wafer or chip in well defined patterns. These patterned layers may be deposited by evaporation masking or photolithography.
In evaporation masking a physical barrier of the appropriate shape is present in close contact with the surface on which the thin film layer is to be deposited. The barrier intercepts defined portions of the evaporant beam. The portions intercepted are prevented from condensing on the surface. The evaporation beam condensate forms the thin film pattern on the surface.
In photolithography, also known as subtractive etching, the thin film is already present on the surface, and the desired pattern is formed in the thin film by selective removal of portions of the film. The removal is accomplished by the use of light sensitive organic lacquers, i.e., photoresists. The photoresist is deposited atop the thin film, and the desired pattern is formed in the photoresist layer by actinic radiation. Development of the photoresist layer causes a significant difference in the solubility of the exposed and the unexposed regions of the photoresist layer. The more soluble region of the photoresist layer is removed by a suitable solvent. In this way, a portion of the underlying thin film is uncovered for the etchant and may be etched, leaving behind the desired surface pattern.
In the photolithographic process the pattern is defined by a stencil or a mask, i.e., a photomask. The mask is a precision image of the pattern to be etched. It is prepared by the 200 fold to 1000 fold photographic reduction of the original art work. The original art work shows the pattern to be formed on the thin layer at a scale readable by the unaided human eye.
In the photolithographic process a polymeric photoresist layer is formd on the thin film to be etched. The photoresist layer is then exposed to actinic radiation through the photomask, e.g., by contact printing. Actinic radiation renders one portion of the photoresist relatively more soluble, and the other portion relatively less soluble. The more soluble portion of the photoresist is removed, e.g., by solubilization with a suitable solvent, uncovering portions of the thin film. The uncovered portions of the thin film are then removed by etching, leaving behind a facsimile or reverse facsimile of the photo mask pattern.
Prior art techniques for formation of the film of the polymeric photoresist have been developed for and are especially adapted to small, rigid elements. As described in the prior art, small, rigid elements are elements having no linear dimension greater then about 3 inches. They are formed on or encompass a rigid, non-flexible element, for example a monocrystalline element, that is substantially free from warpage or bow. The prior art photoresist application techniques include spraying, dipping, spin or whirl coating, and gravure rolling. Most commonly spin coating techniques have been used. In spin coating, also known as whirl coating, a pool of photoresist composition, i.e., a pool of photoresist polymer is solvent, is formed on a surface of the element to be coated. Thereafter the element is rotated at a speed of about 2000 to about 6000 revolutions per minute, whereby to deposit the photoresist composition substantially uniformly across the surface of the small, rigid element. The thickness of the photoresist deposit is a function of the viscosity and density of the photoresist composition, and the angular velocity of the element to be coated.
Spin coating has been found to be unsuitable for both large, non-rigid, flexible elements, and for continuous processes. More particularly, spin coating has been found to be unsuitable for roll to roll processes for the formation of micro circuits on flexible substrates, which substrates are of width greater than about 3 inches, which substrates are drawn from one roll through various deposition processes to a subsequent roll.
Similarly roller coating, also known as gravure coating, has been found to be non optimum for large, flexible, non rigid elements.